Which of the following is NOT a valid data type in composite data types?

Composite data types are structures that can hold multiple values or a collection of values, such as arrays or objects. In the context of programming and data types, valid data types often include primitive types that can serve as building blocks for composite types.

A string is a valid data type that represents text, while a number represents numeric values. Boolean is also a valid primitive data type that represents true or false values. These data types can all be elements of composite data structures.

On the other hand, "undefined" is typically not considered a valid data type in the context of composing composite data structures. It represents the absence of a value or the lack of assignment to a variable, which means it does not hold or represent a concrete type of data on its own. Therefore, when it comes to valid data types in composite data types, undefined does not fit the criteria as it does not convey a specific data type that can be used within a composite structure.